1. Field of the Invention
The present invention relates to a laser light source provided with a quasi phase matching wavelength conversion element that performs wavelength conversion by utilizing the nonlinear optical effect, and an image display apparatus and a processing apparatus employing such laser light source.
2. Description of the Related Art
A ferroelectric crystal having formed therein a periodically poled region is used for a wavelength conversion element that exploits the inversion of nonlinear polarization, an optical deflector that exploits a prism-shaped or lens-shaped inversion structure, and so forth.
As a method for forming a periodically poled structure in a ferroelectric crystal, a method shown, for example, in FIG. 15, is adopted. Specifically, first polarization directions of a ferroelectric crystal 201 are aligned to a given direction, and then, with the use of a power supply 205 and a partially formed electrode pair 204, an electric field in a direction opposite to a polarization direction 202 of the ferroelectric crystal 201 is applied across the electrode pair 204, so that the polarization direction 202 between the electrode pair 204 is inverted forcedly.
According to this method, a periodically poled region 203 in which the polarization direction 202 is inverted to a direction opposite to the polarization direction in the other region (unpoled region 206) can be formed within the ferroelectric crystal 201. Herein, it is preferable that a distance between two electrodes forming the electrode pair 204 (components projected in the polarization direction) is short. It is therefore preferable that the ferroelectric crystal 201 is cut out along a facet almost perpendicular to the polarization direction with the plate thickness of 2 mm or thinner.
Incidentally, the periodically poled region 203 is dependent on a forming position of the electrode pair 204. It is therefore possible to form a periodically poled structure by forming the electrode pair 204 periodically.
A quasi phase matching (QPM) wavelength conversion element (hereinafter, simply referred to as the wavelength conversion element) manufactured by periodically poling a ferroelectric crystal as described above has been studied extensively in recent years.
In particular, by forming a periodically poled structure in a ferroelectric crystal having a large nonlinear optical constant, such as LiNbO3 (lithium niobate), LiTaO3 (lithium tantalate) (hereinafter, abbreviated as LN and LT, respectively), or the like highly efficient wavelength conversion can be realized.
In addition, by adding Mg, Zn, In, Sc, or the like to an LN or LT crystal, it is possible to improve resistances to optical damage, and perform a high power wavelength conversion at room temperature.
Also, as described in JP-A-2000-147584, by applying an electric filed to an Mg-added LN (hereinafter, denoted as MgLN) substrate using periodic electrodes, it is possible to form a fine periodically poled structure with a period of several μm. By adopting the wavelength conversion element thus manufactured, it is possible to realize a compact short-wavelength light source that is capable of converting wavelengths of light, such as light from a semiconductor laser, and is thereby applicable in the fields of printing, optical information processing, and photo-application instrumentation control. In addition, by converting light from a watt-class high-output laser (fiber laser, solid-state laser crystal, and so forth), a watt-class short-wavelength visible light (green and blue) or a high-output UV laser can be achieved. It thus becomes possible to provide a high-output short-wavelength light source applicable to a high-luminance display, processing and exposure, and the like. A wavelength conversion element has been studied extensively, in particular, for a green laser, oscillations of which with the use of a semiconductor laser are difficult.
Herein, an example of an intra-resonator wavelength conversion laser light source employing a wavelength conversion element that generates green light will be explained in reference to FIG. 16.
As shown in FIG. 16, a wavelength conversion laser light source 700 includes a semiconductor laser chip 701, a solid-state laser crystal 703, a wavelength conversion element 705, a holding member 711 that holds the wavelength conversion element 705, and a control device 712 that controls the wavelength conversion element 705.
In FIG. 16, Dc-axis denotes the C-axis direction of the wavelength conversion element 705 and Df denotes a propagation direction of a fundamental laser light 704.
The semiconductor laser chip 701 generates a pump laser light 702 having a wavelength of 808 nm. It is arranged such that the solid-state laser crystal 703 made of Nd:YVO4 or the like and disposed inside a resonator (to be described later) is pumped by the pump laser light 702 to oscillate a fundamental laser light 704 having a wavelength of 1064 nm. The fundamental laser light 704 is converted into a wavelength-converted laser light 710 having a wavelength of 532 nm (green light), which is a second-order harmonic, within the wavelength conversion element 705 provided in the resonator. Then, the wavelength-converted laser light 710 (532 nm) thus generated is outputted to an outside of the resonator.
As the solid-state laser crystal 703, an YVO4 crystal (1-mm long) added with 2 at % of Nd is used herein. The solid-state laser crystal 703 can be pumped efficiently by allowing the pump laser light 702 to go incident thereon in such a manner that a polarizing direction of the pump laser light 702 and the C-axis direction of the YVO4 crystal coincide with each other.
Dielectric multi-layers 706 and 708 are formed, respectively, on the light-incident and light-output surfaces of the solid-state laser crystal 703. Dielectric multi-layers 709 and 707 are formed, respectively, on the light-incident and light-output surfaces of the wavelength conversion element 705. The dielectric multi-layer 706 formed on the light-incident surface of the solid-state laser crystal 703 prevents reflection of the pump laser light 702 on one hand and reflects the fundamental laser light 704 on the other hand. The dielectric multi-layer 708 formed on the light-output surface of the solid-state laser crystal 703 prevents reflection of the fundamental laser light 704. The dielectric multi-layer 709 formed on the light-incident surface of the wavelength conversion element 705 prevents reflection of the fundamental laser light 704 on one hand and reflects the wavelength-converted laser light 710 on the other hand. The dielectric multi-layer 707 formed on the light-output surface of the wavelength conversion element 705 prevents reflection of the wavelength-converted laser light 710 on one hand and reflects the fundamental laser light 704 on the other hand. Consequently, a resonator in the wavelength range of 1064 nm is formed between the dielectric multi-layer 706 formed on the light-incident surface of the solid-state laser crystal 703 and the dielectric multi-layer 707 formed on the light-output surface of the wavelength conversion element 705. Light having a wavelength of 1064 nm is thus laser-oscillated.
Herein, for the solid-state laser crystal 703 and the wavelength conversion element 705, a bulk type with no waveguide structure has been adopted. However, because part of the fundamental laser light 704 and the wavelength-converted laser light 710 is absorbed within the solid-state laser crystal 703 and the wavelength conversion element 705, resonance takes place between two planes (between the dielectric multi-layers 706 and 707) due to the thermal lens effect.
As the wavelength conversion element 705, a 5-mm-long Mg-added LiNbO3 crystal having a periodically poled structure with a period of about 7 μm (hereinafter, denoted as PPMgLN) is used. By disposing the PPMgLN crystal in such a manner that the C-axis (polarization direction) thereof and the C-axis of the Nd:YVO4 single crystal serving as the solid-state crystal 703 coincide with each other, a coincidence condition of the polarizing direction of the fundamental wave and the polarization direction, which is one of phase matching conditions of the PPMgLN crystal, can be satisfied. The PPMgLN crystal is thus allowed to operate as the wavelength conversion element.
In order to achieve enhanced wavelength conversion efficiency and stability, it is necessary to increase the interaction effect by adjusting refractive indices of the fundamental laser light 704 and the wavelength-converted laser light 710 within the wavelength conversion element 705. Because the refractive index is dependent on temperatures, the temperature of the wavelength conversion element 705 is adjusted to a temperature at which the conversion efficiency reaches the maximum (hereinafter, referred to as the phase matching temperature) using the control device 712. It should be noted that the phase matching temperature can be changed as desired by changing the poling period of the PPMgLN crystal.
The phase matching temperature of the wavelength conversion element 705 can be set to suit the installation environment. For example, when the installation environment is close to room temperature, the phase matching temperature is set to about 20° C. to 40° C. In a case where the installation environment is inside the apparatus where heat is generated considerably, the phase matching temperature is generally set to 60° C. to 80° C. A reduction in conversion efficiency of the wavelength conversion element 705 occurs due to temperature distributions in a propagation direction of the fundamental laser light 704 in the wavelength conversion element 705. It is therefore preferable, in general, to fix the wavelength conversion element 705 to the holding member 711 having high thermal conductivity. For the holding member 711, copper that is inexpensive and has high thermal conductivity and aluminum that has a smaller thermal capacity than copper, and so forth are generally adopted.
However, in a case where the periodically poled structure is formed in a ferroelectric crystal and a laser light source employs this ferroelectric crystal as a wavelength conversion element, a problem arises in that an output of wavelength-converted light emitted from this laser light source is reduced over time.